Density and temperature dependent feeding rates in an established and an alien freshwater gammarid fed on chironomid larvae

We compared feeding rates of the well-established, non-invasive amphipod Gammarus roeselii with those of the invasive Dikerogammarus villosus at different prey densities and different temperatures in laboratoty experiments using chironomid larvae as prey. Feeding rates were very variable in both species but higher in the invader species than in the well-established non-invasive species. These results were age independent. At high temperature (18-20 degrees C) and high prey density (>= 1000 prey m(-2)) one large adult D. villosus consumed 6-9 prey day(-1) on average while one G. roeselii consumed 4-7 prey d(-1) and one smaller D. villosus of a length of 8-10 mm consumed 2-5 prey d(-1) while one G. roeselii of the same length consumed 3 prey. Predation rate decreased considerably from high (18-20 degrees C) to low temperature (3 +/- 2 degrees C), from 9 to 4 prey predator(-1) d(-1) in D. villosus and from 7.5 to 2.5 prey predator(-1) d(-1) in G. roeselii. We conclude that both gammarids consumed a biomass in the range of 1/2 to 1/3 to their own biomass and that low predation/feeding rate at low temperature could be a reason why, in some places, indigenous gammarids can re-colonize their former habitats for a short period of time in cold winter and early spring in spite of the presence of the invasive. We further conclude that differences in weight between the species in particular in the experiments with adults can have influenced feeding rates; possibly differences in weight- specific feeding rates are small.